For example, as a compressor provided in a refrigeration circuit for an air conditioning system for vehicles, a wabble plate type variable displacement compressor is known wherein a rotational movement of a swash plate rotated together with a rotational main shaft and supported changeably in angle relative to the main shaft is converted into a wabble movement of a wabble plate, and by transmitting the wabble movement to a piston connected to the wabble plate, the piston is reciprocated. In this wabble plate type variable displacement compressor, because it is necessary to prevent the rotation of the wabble plate connected to the piston, a rotation preventing mechanism of the wabble plate is incorporated. With respect to the rotation preventing mechanism of the wabble plate, various improvements for making the compressor small, improving the durability and the silent performance, facilitating processing, cost down, etc., have been investigated.
For example, in Patent documents 1, 3 and 4, a structure provided with a Birfield type constant velocity universal joint as a wobble plate rotation preventing mechanism is disclosed. In this structure, since wabble parts and a swash plate are supported by an outer ring of a Birfield type constant velocity universal joint provided as a wobble plate rotation preventing mechanism, and ultimately supported by a main shaft via a cage of an internal part of the constant velocity universal joint (a cage for regulating positions of a plurality of balls for performing power transmission), and further, via an inner ring of the constant velocity universal joint, the number of interposed parts increases and the accumulated play becomes great, and therefore, there is a problem insufficient in vibration, noise and durability.
Further, although the Birfield type constant velocity universal joint disclosed in Patent documents 1, 3 and 4 theoretically has a structure performing a rotational power transmission between inner and outer rings by a plurality of balls, actually it is a multiple restriction structure, and it is difficult to achieve uniform and continuous contact of the plurality of balls, and therefore, a contact pressure of specified balls may locally increase. Further, because the rotational power transmission between inner and outer rings is performed in the shear direction of balls by ball guide grooves formed on each of inner and outer rings on both sides of a cage, the contact surface between the balls and the guide grooves may have a large inclination relative to the power transmission direction. By this, when a predetermined power is transmitted, the contact load generated as a vertical reaction force becomes high. Therefore, in order to ensure a sufficient transmission ability, it is necessary to employ a sufficiently large ball size (ball diameter), and from these reasons, it is difficult to make the structure further small-sized, and it is difficult to apply it to a small displacement compressor.
Further, since the support for the rotational main shaft of the compressor in the internal mechanism described in Patent documents 2, 3 and 4 is provided on one side relative to the main mechanism portion (a cantilever supporting is employed), whirling of the main shaft becomes great, and it is disadvantageous on durability, vibration and noise.
Further, in the compression mechanism disclosed in Patent documents 3 and 4, since the inner ring of the constant velocity universal joint is supported slidably in the axial direction at a condition being prevented with rotation, it is necessary to make the main shaft thick in order to ensure the rigidity of the main shaft provided to the housing to be sufficiently great, and it may cause increase of the weight of the main shaft and the weight of the product.
Further, in the constant velocity universal joint mechanism disclosed in Patent documents 3 and 4, machining of grooves for regulating the positions of a plurality of balls operating for power transmission is complicated, and the mechanism may be disadvantageous on cost.
Furthermore, in the compression mechanism disclosed in Patent document 2, since there is no support in the radial direction due to the main shaft in the main mechanism portion and play in the wabble portion in the radial direction tends to become great, by this play, problems on durability, vibration and noise may occur.    Patent document 1: U.S. Pat. No. 5,112,197    Patent document 2: U.S. Pat. No. 5,509,346    Patent document 3: U.S. Pat. No. 5,129,752    Patent document 4: JP-A-2006-200405